This invention relates to a process for the production of compression molded materials using polyisocyanate binders or mixtures of polyisocyanates and other binders together with isocyanate reactive materials and alkylene carbonates.
Compression molded materials, such as chipboard, composite board, or other such molded products, are conventionally produced by hot pressing inorganic or organic raw materials, such as a mass of wood shavings, wood fibers, or other material containing lignocellulose, with various glues or binders. The woodworking industry, which is the largest manufacturer of compression molded materials, still uses what has generally been regarded as the most important binders, including, for example, aqueous dispersions or solutions of urea-formaldehyde ("aminoplast") or phenol-formaldehyde ("phenoplast") resins.
The use of polyisocyanates or polyisocyanate solutions instead of formaldehyde-based resins as binders for pressboard is also known. (see, e.g., German Offenlegungsschriften 1,271,984, 1,492,507, 1,653,177, and 2,109,686.) Polyisocyanates, which have been increasingly used industrially as binders since 1973, improve the stability and moisture resistance and increase the mechanical strength of the products. In addition, polyisocyanate binders have extensive process technology advantages, as disclosed in German Offenlegungsschrift 2,109,686.
The large scale industrial production of materials bonded with polyisocyanates, especially materials containing lignocellulose such as wood chipboard, has, however, been hindered at least in part because, in contrast to materials which are bonded with aminoplast resins, chips bonded with polyisocyanates have no intrinsic tackiness (i.e., no capacity for adhesiveness) at room temperature. Even preliminary compression at room temperature ("cold pressing") of materials bonded with polyisocyanates, but which are still moist, will not yield preforms that are sufficiently stable and self-supporting for the purposes of many production plants. As a result, the universal application of polyisocyanates for the production of compression molded materials is made very difficult.
The molded chips or shavings that are spread over belts, press plates, and the like are transferred to other belts, plates, rolls, or the like on their way to the hot press. These preforms are then discharged from them or the supports are pulled out from under them. In order to enable this process to be carried out without destroying the preforms of chips and without damaging their outer zones, the preforms are subjected to a cold precompression. This precompression is also intended to bond the surface chips together so that as the hot press plates are brought together the air escaping between the preforms and the plates will not carry away any chips (i.e., so that no cavities will form on the surfaces due to the escape of air). Prepresses operating continuously and in cycles are available to subject the chips to specific pressures of up to 40 bar for from 10 to 60 seconds.
One object of the present invention is development of a process which eliminates the disadvantage of the lack of cold tack of chips blended with polyisocyanate binders while at the same time not sacrificing the easy pourability of the bonded chips, a characteristic necessary for spreading chips easily into the required shapes of the preforms. The process according to the invention solves this problem in a manner which is surprisingly simple for one skilled in the art.